This invention relates to mixing chemicals in a fluid stream and more particularly to hydraulic diffusion flash mixing of chemicals in water treatment and waste water treatment.
Chemicals have long been used in the treatment of water and waste water. In instances of treatment practice it is possible to identify the application of chemicals in the form of gases, liquids and solids. In all cases it is advantageous to achieve a uniform dispersal of the chemicals into the water stream as rapidly as possible and to ensure that chemicals that have already been dispersed do not return to the region wherein fresh chemicals are being introduced into the water stream. Meeting this last condition requires that the dispersal and mixing of the chemicals be done in a plug flow regime.
A first class of chemicals applied in the liquid form in water and waste water treatment is coagulants. They are used to induce flocculation of particles suspended in the raw water to be treated. This aggregation of suspended particles allows for more efficient sedimentation and/or filtration downstream. For best results, the initial mixing of the chemical coagulant with the raw water should occur as rapidly as possible to form a homogenized mixture within a second, or less.
The principal objective of this rapid or flash mixing in coagulation practice is to ensure a homogeneous coagulation by completely uniform dispersal of the coagulant throughout the water. In this way, the coagulant can make contact with the maximum number of suspended particles prior to the completion of hydrolysis, enabling intermediate complexes to destabilize the suspended particles initiating aggregation. This chemistry of destabilization sets some of the requirements for efficient rapid mixing.
Chemical coagulants should be dispersed in an unblended stream of raw water. Dispersing chemicals into a blended or partly blended stream (backmixing) can lead to poor destabilization of a fraction of the particles because some might have insufficient surface coverage while others might have too extensive surface coverage by adsorbed chemical species. This wastes chemicals and results in less effective floc formation for a given amount of a coagulant.
Stagnation time, defined as the amount of time that elapses from the addition of a coagulant to the start of mixing, should be reduced for the most effective coagulation. Experts on coagulation suggest that a sufficiently short stagnation time is achieved with a hydraulic jet mixer if the jet plume can be made to cover the entire cross section of the mixing conduit within one conduit diameter downstream of the mixer location when the conduit is flowing at its maximum capacity.
A second class of chemical used in water and waste water treatment includes those associated with disinfection. The primary concern in water and waste water treatment is the elimination of water born disease. Suspended particles are removed from potable water primarily because they can interfere with disinfection by shielding pathogens from contact with biocides. The aesthetics of suspended particle removal is a secondary concern.
The chemicals used in disinfection practice are chlorine, ammonia and ozone. Chlorine and ammonia can be fed as a gas or as a solution in water. Ozone is fed as a gas. In all cases, the concerns about short stagnation time, even dispersion, and avoidance of backmixing that have been enumerated in the discussion of coagulation apply to the disinfection process. When a disinfectant is fed as a gas it is also necessary to ensure efficient transfer of the disinfectant from the gas phase to the liquid phase by minimizing the size of the gas bubbles. Small bubbles are preferred because a disinfectant molecule has a shorter distance to travel from the interior of the bubble to the gas-liquid interface, and because the ratio of surface area to volume is larger for small bubbles. Both of these considerations improve the gas to liquid mass transfer process.
A third class of chemicals is used in water treatment to eliminate unpleasant taste and odor. Most often potassium permanganate and activated carbon are used for this purpose. Potassium permanganate is fed as a solution in water. Powered activated carbon (PAC) is fed as a slurry. When activated carbon is used in the granulated form, the water to be treated is passed through fixed beds of the granulated material. Rapid mixing is used to feed potassium permanganate and PAC and the concerns about short stagnation time, even dispersion, and avoidance of backmixing that have been enumerated in the discussion of coagulation apply to the elimination of taste and odor by the use of these chemicals.
From a mechanical point of view, a rapid mixing device should be simple, practical, and relatively inexpensive and should not create appreciable head loss.
Through the years, in attempting to meet theses chemical and mechanical requirements, many devices have been employed to provide rapid mixing needed for chemical dispersion. These include the weir, the Parshall Flume, and rapid mixing chambers equipped with mechanical rotary mixing devices such as propellers or turbines and in-line blenders. More recently, hydraulic diffusion flash mixing has been used as a method providing rapid mixing without appreciable head losses and lower operating and maintenance costs than mechanical methods. This method also provides more efficient rapid mixing with reductions to 20 to 50 percent in chemical consumption over mechanical methods.
Generally hydraulic diffusion flash mixing operates by drawing off a portion of the water to be treated into a carrying water loop. The chemical to be dispersed is added to this drawn-off portion. The mixture of carrying water and chemical is then injected into the remainder of the water through a diffuser. A pump in the carrying water loop provides the pressure for injection.
Usually the diffuser is a radial diffuser which injects the carrying water and the chemical mixture perpendicular to the flow direction of the remaining water from a deflector plate or from several nozzles equally spaced about the circumference of a tube placed in the center of the conduit carrying the remaining water. Radial injection can also occur by injecting perpendicular to the flow direction from nozzles equally spaced about the pipe periphery. In theory, this alternative reduces head losses, but is more difficult to construct, so central injection is preferred.
In other versions of hydraulic jet diffusion, the jet nozzles are placed on a tube that crosses the major diameter of the conduit carrying the remaining water; or on a grid of tubes that crisscrosses the conduit carrying the remaining water. These jet nozzles can be situated so that they discharge perpendicular to the direction of flow in the conduit, or either upstream or downstream to the direction of flow. These versions can cause objectional head losses in the conduit carrying the remaining water. The multiplicity of nozzles that are required to attain a short stagnation time, even dispersal, and avoid backmixing, require that the chemical be mixed with a relatively large amount of carrying water, That is inefficient in the amount of power required by mixing and requires the use of more chemical because the large dilution by carrying water reduces the efficiency of coagulation and can cause nozzle clogging by increasing the propensity of the coagulant to precipitate. Thus, central injection is preferred.
Sometimes the diffuser is a conical jet diffuser which injects carrying water and chemical mixture parallel to the flow direction of the remaining water through a single nozzle, directed either upstream or downstream with the flow, located in the center of the conduit carrying the remaining water. Both directional options are versions of the central injection scheme. Because flow through a conical nozzle requires more power than the convergent nozzle used in the radial jet versions, and because the water leaving the conical nozzle does not flow entirely perpendicular to the direction of flow of the remaining water, thus causing a degree of backmixing, and increasing stagnation time, the radial jet diffuser is preferred over the conical nozzle option.
Problems have developed with hydraulic jet diffusion mixing on some applications. Where hardness exists in the water under treatment, the addition of some chemicals, in particular coagulants and chlorine in the carrying water loop has lead to clogging of the diffuser nozzles. This clogging requires periodic plant shutdowns to clean the diffuser, resulting in greatly increased operating and maintenance costs.
A hydraulic diffusion flash mixing system in which chemicals are directly introduced into the carrying water flow is disclosed in U.S. Pat. No. 4,869,595 to Lang. In this system water to be treated flows in the main conduit, and a portion of this water is diverted and reintroduced into the main conduit by a narrow auxiliary pipe. The auxiliary pipe""s outlet is formed by numerous small nozzles around the periphery of the auxiliary pipe for injecting carrying water perpendicular to the flow direction in the main conduit. A chemical feed pipe leads to a manifold positioned around the auxiliary pipe adjacent to the nozzles. The manifold has its own nozzles which surrounds the auxiliary pipe and inject chemicals in the direction of the main conduit water flow, i.e., perpendicular to the direction of the carrying water flow so that the chemical and the carrying water flow mix and at the same time mix with the main conduit water flow. However, the numerous injection nozzles create a relatively complex structure and are not immune from clogging due to particulate impurities in the injected chemical. This is so because of the relatively small volume of the chemical flow in relation to the treated water flow (on the order of a million times less), and because the chemical nozzles must be small enough so that the headloss through them is high enough to ensure that the chemical is properly dispensed through each chemical nozzle.
A refinement of the hydraulic diffusion flash mixing system disclosed in U.S. Pat. No. 4,869,595 to Lang, wherein the multiplicity of carrying water nozzles is replaced by a single carrying water nozzle and the multiplicity of chemical nozzles is replaced by a single chemical nozzle is disclosed in U.S. Pat. No. 5,183,335 to Lang et al. In this system water to be treated flows in the main conduit, and a portion of this water is diverted and reintroduced into the main conduit by a narrow auxiliary pipe. The auxiliary pipe""s outlet is formed by a single nozzle for injecting carrying water either upstream or downstream to the flow direction in the main conduit. The carrying water jet impinges on a conical deflector plate who""s axis of rotation is coaxial with the centerline of the carrying water jet nozzle and is located a short distance from the open end of the carrying water jet. The conical deflector plate turns the flow of carrying water perpendicular to the flow of water in the main conduit. In one variant of this system a chemical feed pipe leads to a poppet valve located at the apex of the deflector plate cone. In the other variant of this system a chemical feed pipe leads to an injector pipe that runs through the carrying water nozzle and is located so that the injector pipe""s centerline is coaxial with the center line of the carrying water nozzle and the end of the injector pipe is located between the end of the carrying water nozzle and the apex of the deflector plate cone. In both variants of this system the chemical and the carrying water mix as they pass in contact with the deflector plate and then mix with the main conduit water flow. However, the injector pipe requires a supporting spider located inside the carrying water nozzle. This is costly to manufacture and is vulnerable to clogging if there are large particulates in the carrying water flow. Furthermore, the poppet valve is not suitable for the injection of gases or slurries.
The present invention promotes the efficient mixing of a chemical with water not only by directly injecting the chemical into a main water stream, but also by subjecting the chemical to turbulence created by a water supply pipe having a jet nozzle that supples a high-velocity water stream at the point where it is injected into the main water stream. More specifically, water to be treated flows through a main conduit, and a secondary pipe and the high-velocity nozzle injects a secondary flow of carrying water into the main water flow and in the same, or opposite, direction as the main flow. A deflecting device positioned downstream of the jet nozzle intersperses the high-velocity flow with the main flow by deflecting the high-velocity flow radially outward in all directions. Chemicals pass through a small supply line into the deflecting device and exit at, or adjacent to, the tip of the deflecting device in a direction perpendicular to the direction of the high-velocity jet. Chemicals mix with the high-velocity jet as they pass over the surface of the deflecting device and then mix within the main flow after they have passed over the deflecting device.
In an embodiment preferably suited for dispersal of liquids and slurries, the deflector""s nozzle is a cylindrical cavity with a center line coaxial with the center line of the deflector. The opening of the deflector""s nozzle, where chemicals are discharged, forms the apex of the conical deflector. Moreover the shape of the cylindrical cavity that forms the deflector""s nozzle is preferably not that of a right cylinder, but flared like the end of a trumpet, the large end of the nozzle being the point where chemicals are first contacted by and mix with the high-velocity jet. The precise shape of the end of the deflector nozzle is a function of the density and viscosity of the chemicals it carries, the flow rate of those chemicals, and the velocity of the high-velocity jet. In general the bell shape must be determined for each particular application of this embodiment. The means of determination are preferably a numerical modeling process using a software program designed for that purpose. Such programs are generically known as Computer-aided Fluid Dynamic (CFD) programs and are available to the general public.
In an embodiment preferably suited for dispersal of gases, the deflector""s nozzle is a cylindrical cavity with a center line coaxial with the center line of the deflector. The opening of the deflector""s nozzle where gaseous chemicals are discharged, are ports on the surface of a right cylinder with an axis coaxial with the axis of the deflector, and located adjacent to the apex of the deflector cone. An educator tip is fitted to the apex of the deflector cone. The educator tip has a base diameter that is larger than the diameter of the deflector at the point of the educator tipxe2x80x2 attachment. Moreover the base of the educator tip has a curvature such that a portion of the high-velocity jet passing over the apex is drawn inwards toward the axis of the deflector cone by the tendency of a flowing fluid to adhere to a surface. The gas is drawn outwards through the ports by the vacuum created by the high-velocity jet. The inward flow of a portion of the high-velocity jet shears the outward flow of gas into a multiplicity of small bubbles, usually with a diameter of less than 1 millimeter. These bubbles are of a size that is highly efficient for the mass transfer of chemicals from the gas phase to the liquid phase. The exact shape of the curvature of the base of the educator tip will vary with the velocity of the high-speed jet and the dimensions of the deflector cone. In general the curvature must be determined for each particular application of this embodiment. The means of determination being preferably a numerical modeling process using a CFD software program designed for that purpose.